Transition metals are fast diffusing species in Si and readily incorporated during various stages of device processing. The impurities and metal-related defects may act as recombination centers in devices, and thereby cause low yields and poor reliability. Efforts undertaken to reduce the effects of metal contamination have involved two approaches: 1) removal of device degrading impurities from the active device regions by gettering and 2) deep-level passivation by hydrogenation.
Gettering processes are divided into two categories: intrinsic and extrinsic. Intrinsic gettering is based on the fact that under proper conditions supersaturated oxygen in silicon wafers will precipitate, and the stress resulting from precipitation will induce stacking faults and dislocation loops. These dislocations become sites at which unwanted impurities can be trapped and localized. Extrinsic gettering involves the use of external means to create the damage or stress in a silicon lattice that leads to the creation of the extended defects or chemically reactive sites at which the mobile impurities are captured. Extrinsic gettering has been accomplished by utilizing phosphorus diffusion and backside damage introduced by abrasion or ion implantation. High-temperature anneals in chlorine have also been utilized to enhance minority carrier lifetimes. Possible mechanisms include formation of volatile metal chlorides allowing metal ion removal from wafers.
Considerable recent interest has been shown in the passivation of electrically active defects associated with metal contaminants by reaction with atomic hydrogen. Typically the passivation of impurities is achieved by exposing the Si wafer to a low pressure (0.1-0.5 Torr) RF hydrogen plasma at 100-400.degree. C.
Although gettering and passivation with atomic hydrogen are used to reduce detrimental effects of metal impurities on devices, they have their own limitation. For instance, iron is the most common and detrimental impurity, but the species is only weakly gettered by typical wafer getting and remains almost intact upon hydrogenation.
The gettering by chlorine is generally carried out at a temperature above 800.degree. C. in order to convert a deleterious metal to the metal halide and to volatilize the halide at the exposed surface. The effectiveness of the method is largely dependent on the volatility of the metal chlorides. Moreover since chlorine is immobile, the method is not applicable to a large number of metal species which have low diffusion coefficient at the processing temperature. The corrosive nature of HCl is another drawback and led to consideration of other Cl containing materials as alternative. These gases, however, introduce their own problems. For example, TCE is a known cancer causing agent, while TCA reacts to form phosgene, COC1.sub.2, a deadly gas.
In contrast to chlorine, hydrogen is extremely mobile, and thus hydrogenation can be carried out at relatively low temperatures. However, the passivation process is not stable, as indicated by the observation that subsequent heating a passivated wafer at temperatures above 400.degree. C. causes a reappearance of the defect states because of the unstable nature of the hydrogen-impurity pairing. It has also been reported that hydrogen not only passivates the detrimental impurities, but also deactivates shallow dopants. For instance, the spreading resistance of a boron-doped Si wafer increases dramatically when the wafer is exposed to atomic hydrogen. It is, therefore, not desirable in Si integrated circuits.
From the foregoing discussion it should be apparent that there is a need within the art for improved techniques for passivating metal impurities within semiconductor materials.